1. School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, MRC Molecular Enzymology Laboratory, The Babraham Institute, Babraham, Cambridgeshire CB2 4AT, U.K., and Program in Apoptosis and Cell Death Research, The Burnham Institute, 10901 North Torrey Pines Road, La Jolla, California 92037